For use in back-side treatment of pressure-sensitive tapes and formation of separators for the surface protection of pressure-sensitive adhesive layers, and the like, various kinds of release agents have been proposed so far which include a silicone type, a long-chain alkyl type, and the like. A release agent for use in the above purposes is required to give a release film having an adequate balance between adhesion and releasability. That is, since a release film is bonded to the surface of a pressure-sensitive adhesive layer in an easily strippable manner, adhesion and releasability should be balanced both at the interface between the release film and its substrate and at the interface between the release film and the pressure-sensitive adhesive layer.
However, release films obtained from any of the conventional release agents have been defective in that they have a poor balance between adhesion and releasability. In the case of pressure-sensitive tapes, for example, there have been the following problems. If the adhesion of the release film to the pressure-sensitive adhesive layer is too strong, the pressure-sensitive tape poses a problem that when the tape is rolled up and the roll thereof is stored for a long period of time or under high temperature conditions, the roll becomes unable to rewind. If the adhesion to the pressure-sensitive adhesive layer is too weak, the roll has a problem that the pressure-sensitive adhesive layer slides on the release film and, as a result, the pressure-sensitive adhesive layer is partly exposed and fouled. On the other hand, if the adhesion of the release film to its substrate is too weak, this poses a problem that the release film peels off its substrate, i.e., from the back side of the tape, and is transferred to the pressure-sensitive adhesive layer.
The conventional silicone-based release agents have also had a problem that since cured films obtained therefrom repel oil-based inks, writing such as addressing on the cured films with an oil-based ink is impossible.
In an attempt to develop a silicone-based release agent which forms a cured film on which writing with an oil-based ink is possible, a release agent comprising a blend of a special polyorganosiloxane and ethyl cellulose has been proposed (JP-B-60-1899 and JP-A-59-147048). (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application", respectively.) This release agent, however, has had a problem that it is unsuitable for non-solvent coating because it should be used as a dispersion in an organic solvent in order to obtain the viscosity and homogeneity required as a release agent.
Under these circumstances, the present inventors previously proposed a silicone-based release agent which forms a cured film on which writing with an oil-based ink was possible (JP-A-2-129219).
Into the above silicone-based release agent proposed by the present inventors, fine particles of a silicone rubber elastomer have been incorporated for the purpose of imparting oil-based ink printability to cured films obtained from the release agent. However, this release agent has had a drawback that it is difficult to improve the adhesion strength thereof probably because the release agent is of a silicone type. It has also been found that the proposed release agent has a problem that cured films obtained therefrom are unsatisfactory in oil-based ink printability, or in the property of fixing printed oil-based inks of some kinds, probably because the fine particles of a silicone rubber elastomer contribute only to an improvement in wettability by an oil-based ink. Illustratively stated, cured films obtained from the above release agent have been found to have problems that repelling is apt to occur when oil-based inks containing certain kinds of solvents, e.g., alcohol-type solvents, as main components are applied on the cured films, and that penetration of oil-based inks into the cured films is insufficient and, hence, characters formed by printing oil-based inks on the cured films are apt to fall off by abrasion, etc.